1. Field of the Invention
The present invention relates to calibration techniques that stabilize variations of color in output image due to individual differences of printers or environmental changes, such as temperature-humidity variations. More particularly, the present invention relates to a calibration target curve generation method.
2. Description of the Related Art
Printer calibration stabilizes (compensates for) the tone reproducibility of a printer that varies in accordance with variations in an establishment environment (temperature, humidity, etc.) or aging of the printer. A calibration curve can be obtained by computing a function that compensates for differences between input/output data characteristics with ideal tone reproducibility (a calibration target curve) and the measured values of outputs (a chart including a plurality of color patches) of the actual printer.
The calibration target curve used for calibration of the printer and the like is predetermined or created by an operator.
To be specific, when the target curve is edited, the desired output density values for corresponding input data representing the gradations of, for example, CMYK (Cyan Magenta Yellow Black) are specified to define the target curves.
FIG. 3 shows an example edit screen for generating target curve data during a known calibration. In practice, the domain of input data values x and the range of output density values y are determined for each of CMYK where the input data values (when the data is represented with 8 bits) range from 0 to 255 and the output density values y range from 0 to the maximum density value. For simplicity, the description will be made with each of the values ranging from 0 to 1.
The target curve is obtained as follows: the output density values versus the input data at equal intervals as shown in FIG. 3 are specified; segments between points indicating input-output relationships are interpolated. FIG. 4 shows the target curve specified in FIG. 3.
In the above method of specifying the output density values versus the input data values, when gradient variations in the target curve are steep between the points representing the input-output relationships (for example, between 0.0 and 0.1, between 0.1 and 0.2, between 0.8 and 0.9, between 0.9 and 1.0), errors due to curve interpolation in the corresponding segments increase. In particular, when the load on the operator is intended to be alleviated by decreasing the number of specifying points indicating the input data values (for example, the input data values are specified with an interval of 0.2), more errors occur due to the curve interpolation.